This invention relates to electrophotographic reprographic methods and apparatus. In particular, this invention pertains to improvements to optical projection method and apparatus used to create latent electrostatic images of originals sought to be reproduced.
Image quality in electrophotography, photography and printing arts is a function of the resolution power of the medium in which an image is created. Current day electrophotographic equipment produces high quality images that contain detail resolved at about or above 500 line pairs per inch. This is generally considered high resolution for electrophotography whereas a resolution of about 100 line pairs per inch or below is generally considered as low resolution.
Besides resolution, the density range that a medium can reproduce also effects image quality. A wide density range is desired when the object to be copied contains a wide range of densities. Herein, density is used to refer to difuse reflection density for a white background but should also be understood to include spectral reflection density and transmission density.
When color originals are being reproduced, the faithfulness of the color reproduction affects image quality or acceptability. Without color correction techniques, a color process cannot reproduce well an original made from the reproduction process.
Color and tone corrections are made in the color photograph and printing arts, among other ways, by laser scanning techniques. Typically, an original is scanned point by point by a photocell in a line by line raster pattern. The individual color content and tone, i.e. density, of each point or pixel is represented by an electrical signal which is evaluated by a computer and both color and tone corrected. The corrected electrical signal is then used to modulate the intensity of a spot of light moved point by point in a raster pattern over a photographic emulsion. The correction is made directly to the image or video signal, as such the correction is analogous to a multiplicative color correcting mask in the color graphic arts. The emulsion is generally a very high resolution medium being able to resolve as high as 1500 line pairs per inch. The size of the scanning spot is generally limited however to dimensions that reduce the process resolution to about 1000 line pairs per inch. There are no lenses used to project a light image to the emulsion. By using only a scanning spot exposure of the emulsion, there is no need to register a scanned image with an optically projected image.
The Xerox 6500 Color Graphite Printer (CGP) is a Xerox 6500 color machine modified to include a laser scanner for exposing the machines photoconductive drum. However, the laser exposure mechanism is fed by video signals generated independently of an original on the platen of the copier. There are no presently known electrophotographic machines, either color or black and white (monochrome), that use a raster scanner in parallel--i.e. in an additive mode--with lamp and lens projection optics to make a color or tone corrected copy.
The composition of an original to be copied also affects the quality of a copy. Specifically, electrophotographic machines have difficulty accurately reproducing continuous tone originals because of the high contrast inherent to the electrophotographic processes. Also, if the machine has a half-toning screening mechanism, it has difficulty reproducing half-tone originals. The rescreening creates beat frequencies in the copy that may be objectional to a human observer. The present invention includes an input raster scanning mechanism that is most applicable to producing copies of continuous tone originals.